The present invention is in the field of dishwasher detergents. In particular, the present invention relates to dishwasher detergents which comprise zinc salts.
With the continuing automation of very diverse washing and cleaning processes domestically and in industry, machine washing and cleaning compositions for textiles and dishes have become increasingly important in the past decades.
The so-called low-alkaline detergents required for machine dishwashing often comprise, as alkali carriers, mixtures of sodium disilicate and soda, builders such as citric acid, for example in combination with polycarboxylates, and preferably low-foam, nonionic surfactants. In addition, bleaches, bleach activators, silver protectants and corrosion protectants and, to enhance the detergency, enzymes may be present. In a typical dishwasher cycle, the dishes placed into baskets are cleaned as a result of intensive contact with the aqueous detergent solution at about 65° C. and pH values between 9 and 11 and are then rinsed clear.
An important criterion for assessing a dishwasher detergent is, as well as its detergency, the optical appearance of the dry dishes after washing. Any calcium carbonate deposits which arise on dishes or in the inside of the machine can, for example, adversely affect customer satisfaction and thus have a causal influence on the economic success of such a detergent. A further problem which has been in existence for a long time with machine dishwashing is the corrosion of glassware, which may usually manifest itself in the appearance of clouding, streaking or scratching, or else by iridescence of the glass surface. The observed effects are based essentially on two processes, the escape of alkali metal and alkaline earth metal ions from the glass combined with hydrolysis of the silicate network, and secondly deposition of silicatic compounds on the surface of the glass. To avoid such corrosion processes, the prior art gives a series of proposals, for example with regard to the use of various silicates.
For example, international patent application WO 96/12783 (Henkel KGaA) describes phosphate-free to low-phosphate dishwasher detergents with improved decoration protection and glass protection based on citrate-containing formulations which comprise crystalline layered silicates.
International patent application WO 99/57237 (Clariant, Henkel KGaA) provides phosphate-containing dishwasher detergents which comprise a pulverulent to granular additive which have, as essential constituents, a crystalline layered silicate of the general formula NaMSixO2x+1.yH2O, in which M is sodium or hydrogen, x is a number from 1.9 to 22 and y is a number from 0 to 33, and (co)polymeric polycarboxylic acid and, as well as having glass and decoration protective effects, also have excellent detergencies.
However, the use of zinc or zinc salts for preventing glass corrosion during machine dishwashing has also been described.
According to the teaching of the American patent specification U.S. Pat. No. 3,677,820 (Whirlpool), a zinc strip attached to the inside of the dishwasher prevents, for example, the corrosion of glass surfaces during the washing operation.
Finally, European patent application EP 0 383 482 (Procter & Gamble) describes dishwasher detergents comprising insoluble zinc salts which are characterized by improved glass corrosion protection. To achieve such an effect, the insoluble zinc salts must have a particle size below 1.7 millimeters.
International patent application WO 00/39259 (Reckitt Benckiser) discloses water-soluble glasses in accordance with DIN ISO 719, which comprise at least one glass corrosion-inhibiting active ingredient whose weight fraction in the glass is not more 85% by weight and which is released from this glass under the conditions of the wash and/or rinse cycle.